Gable-top containers or containers are used widely for packaging milk, juices, and other liquid foods, as well as a variety of other food and non-food products. Such containers are often made from sheet material which is heat-sealable to itself. A typical material for gable-top containers is paper board coated on both sides with polyethylene (typically LDPE) or other heat-sealable material.
Completed gable-top cartons are typically adapted to be resealably opened along a top seal or fin. However, it is not uncommon for the heat seal between the LDPE layers to be stronger than the paper board substrate itself. Often, when the carton is opened, the paper tears away from the LDPE layers, or "delaminates", along the seal area.
Delamination usually occurs on or within the mouth of the spout, which is in or near contact with the contents of the package when the contents are poured out. The fibrous, torn surface of the spout is unsightly, and can be unsanitary if food in the container collects on the torn surface as the container is emptied.
In some gable-top cartons, abhesive areas are provided to reduce area of the heat-sealed surfaces. When abhesives are used, the sealing areas are reduced without requiring the sealing machine to narrowly focus the heat-seals, and delamination is greatly reduced.
One problem with conventional abhesives is that they hinder or prevent the formation of a proper fluid seal. This is of particular importance when the material to be packaged requires an hermetic seal, which reduces the transfer of oxygen and other gases, as well as liquids, through the sealed area. In known packages, a partial heat seal is required in addition to the abhesive seal, in order to affect hermetic sealing. The problem of delamination thus is confined to a smaller area, but not eliminated with conventional abhesives.
The conventional gable-top container also presents other functional or manufacturing problems. To allow proper container sealing, the abhesive areas must be precisely placed to leave a sealable margin. The thickness, viscosity, and choice of the abhesive must also be controlled so the abhesive will be effective within its intended boundaries without migrating and acting outside of those boundaries to any significant degree.
The need for precise application of a partial coating such as an abhesive on a substrate makes the application more expensive and difficult, or subjects the filled containers to a higher than desired seal failure rate. Reduction of the abhesive areas to provide a greater margin for migration and variable application increases the size of the torn heat-sealed surface.
The need for abhesives has also been reduced by confining the heat sealing region closely to parts of the inner surface of the carton, instead of (or in addition to) using the abhesives. Again, however, it is difficult to precisely confine ultrasonic, radiant, or conductive heating with a high degree of accuracy and reproducibility, so either the cost and difficulty of operating such equipment increases or the ability to provide a container with highly reproducible opening characteristics suffers.
If conventional polyethylene-coated heat-sealed containers are sealed in confined areas without using abhesives, the sealing temperature must be maintained within a range of a few degrees Fahrenheit (less than 2.degree. C.) to provide sealing integrity without rendering the sealed container difficult to open or subject to delamination. This narrow temperature range is difficult to maintain in production machinery. Deviations from this range result in over- or under-sealed containers. The containers must be over-sealed to some degree to ensure that all are adequately sealed.